This invention relates in general to means for varying the speed of an AC (alternating current) motor, and in particular to means for varying the frequency and amplitude of the power supplied to the motor.
Large volume submersible pumps typically are located thousands of feet into a well. The pump assembly includes a centrifugal pump which is driven by an AC motor mounted below it. The motors may range from 15 to 750 horsepower, thus require a large supply of power. Normally, 60 cycle, three phase power is supplied, with voltage phase-to-phase being 480 volts or more. Common rotational speeds of the motor are about 3500 rpm (revolutions per minute).
Most of these types of pumps are single speed pumps. Because of different viscosities, densities, well flowing characteristics and the like, it would be desirable to vary the speed of the motor.
One way in which to vary the speed is to vary the frequency of the power being supplied. Normally, the line power comes from a utility company, and cannot be changed from the standard 60 cycle per second. There are circuits that will convert the standard frequency to different frequencies. These circuits also change the amplitude in proportion to the frequency change for efficient operation of the motor. In the past, SCR circuitry (silicon controlled rectifier) has been used. One disadvantage of an SCR is that it will not switch off until the current drops to zero. This requires complex commutating circuitry, making these control systems expensive.